My understanding is that it's the diesel engines that don't like it (the generator head itself is fine). Basically, some of the fuel in the combustion chamber doesn't get burned, and comes out the exhaust. This builds up in the exhaust system, forming an ooze in the pipes. I think it's worse on a turbo diesel, because now the ooze is coming into the turbocharger.

https://en.wikipedia.org/wiki/Wet_stacking